Spatial light modulators (SLMs) such as digital micro-mirror devices (DMDs), liquid crystal display (LCD) and liquid-crystal-on-silicon (LCoS) devices are often used to project images in projection systems. The optical projection systems can include televisions, video displays, and projectors. The optical projection systems are usually formed in two parts, which are: an illumination system for generating and collecting the light rays needed to illuminate an image, and a projection system for collecting the illuminated image light rays into a projection lens which projects the final image. At least one spatial light modulator receives the illumination light rays from the illumination system, modulates the light received with image data for visual display, and transmits image light rays for projection to the projection system. In many systems, the spatial light modulator is an array of reflective pixel elements. SLMs can be DMDs that receive digital image data and reflect (or do not reflect) the illumination light, forming an image corresponding to the digital image data.
Projection systems have been implemented with DMDs with relatively small pixel pitch, such as DMDs with a pixel pitch of less than 7 microns. For example, a DMD device currently available from Texas Instruments Incorporated has up to 2 million individually addressable pixel mirror elements in a two dimensional array. Systems incorporating these new, smaller devices can increase some visible artifacts due to potential interactions between the pixel elements and the illuminating light. A DMD with a small pixel pitch of less than about 7 microns operates as a diffraction grating for the wavelengths of illuminating light being used. This interaction can produce visible artifacts in the displayed image. Also, the dithering applied to produce precise gray scale outputs between the directly producible gray shade levels in these systems can create additional visible diffraction artifacts. Although the examples described herein are described with reference to DMDs used as SLMs, the same interaction can be found in other SLM devices, such as LCoS devices.
Diffraction artifacts are particularly visible across shallow gradients in a displayed image. Dithering techniques are used to create gray scale renditions for shades that fall between the limited numbers of gray shades that the system can directly produce with pulse width modulation. By alternating between the reproducible gray shades, the desired gray shade is created by “dithering” between these gray shades. When the pixel pitch is reduced to these new small sizes, the dithering techniques can also operate as interference patterns for the wavelength of visible light. The gray shade that is visible is then either brighter than expected (due to constructive interference) or darker than expected (due to destructive interference).